1. Field of the Invention
The present invention relates to an image forming apparatus for shifting a sheet along a sheet width direction that is perpendicular to a sheet transporting direction before the sheet is transported at an image-forming position with the sheet being nipped.
2. Description of Related Art
An image forming apparatus having multiple functions which have functions of a printer, a scanner, a copier, a facsimile and the like combined together has been widely used in recent years. In the image forming apparatus, there may be a deflection in which when transporting the sheet from a paper feeder or a reversing path to a secondary transfer unit during a period of image-forming time, the sheet is deflected to a direction (hereinafter, also referred to as “sheet width direction”) perpendicular to a sheet-transporting direction of the sheet because of any mechanical factors in the apparatus. As the mechanical factors in the apparatus, for example, any errors in manufacturing the rollers and/or any wear on long-term deterioration are illustrated. When performing a printing operation while there is such a deflection of the sheet, an image is printed on the sheet while it is deflected from an image forming position thereof.
In order to align the image and the image forming position of the sheet accurately by taking the deflection of the sheet into consideration, a registration roller shift correction is carried out by which the deflection of the sheet can be corrected by shifting the sheet to the sheet width direction with the registration roller nipping the sheet. For example, Japanese Patent Application Publication No. 2007-22680 has disclosed an image forming apparatus in which a registration roller is arranged at an upstream side from an image forming position and a line sensor is arranged at a downstream side of the registration roller as well as the deflection of the sheet can be corrected by shifting the sheet to the sheet width direction based on the deflection amount of the sheet detected by the line sensor.
Here, the following will describe a configuration of a general shift mechanism for shifting the sheet using the registration roller. FIG. 1 shows an outline configuration example of an important portion of such a general shift mechanism 300, which is seen from a side thereof. As shown in FIG. 1, a driving roller 32 constituting the registration roller 30 is provided with a shift mechanism 300 for shifting the registration roller 30 to a sheet width direction D2. The shift mechanism 300 contains a rack 302 attached to an end of the driving roller 32, a round pinion 304 that is arranged to engage with the teeth 302a formed on a side surface portion of the rack 302 and a motor, not shown, that drives the pinion 304 to rotate.
When driving the motor, the pinion 304 rotates. The driving roller 32 fixed on the rack 302 is then shifted by rotation of this pinion 304 to the sheet width direction D2. The rack 302 is biased under any predetermined urging force F by a spring 306 to a front side (frontward direction) of the image forming apparatus to maintain the shift of the registration roller suitably. The registration roller 30 stays at a home position HP thereof under normal conditions in which the registration roller 30 is not shifted.